Peak bone mass, which is achieved soon after the end of sexual development, is the most important determinant of bone mass and osteoporosis later in life. Disabled children, such as children with cerebral palsy, are particularly vulnerable to deficits in bone mass accretion due to decreased mobility and weight-bearing which reduces mechanical loading of the skeleton. In addition, these children have poor muscle strength and function which contributes to the lack of mechanical stimulation needed to build bone mass. Physical therapy can increase bone mass and improve muscle function, but the therapy is time and labor intensive and may not be available as frequently as needed. Whole body vibration has shown promise as an alternative method for stimulating both increases in bone mass and improvements in muscle performance. In whole body vibration interventions, the subject stands on a platform that vibrates either up and down or in a rocking motion. The main purpose of this proposal is to test the efficacy of high frequency, low magnitude vibration as an intervention for low bone mass and poor muscle function in children with cerebral palsy. Specifically, this study will test the following hypotheses: (1) The vibration intervention will increase axial and appendicular bone density and long bone cross-sectional properties relative to controls; (2) The vibration intervention will increase muscle mass, muscle strength, and postural stability relative to controls; (3) The vibration intervention will decrease markers of bone resorption and increase markers of bone formation relative to controls. In the vibration intervention, subjects will receive mechanical stimulation for 10 rain/day, 7 days/wk for 6 months. These subjects will undergo an additional 6 months of weight bearing without vibration following the same schedule to serve as an internal control. An additional control group of matched children will receive no intervention. Changes in lumbar spine and proximal tibia cancellous bone density, cross-sectional properties of the midshaft of the tibia, markers of bone turnover, calf muscle cross-sectional area and strength, and indices of postural stability will be measured to determine whether or not the vibration intervention is effective in improving bone mass and muscle function. Using CP as a model, this study will help to validate or invalidate the general usefulness of low magnitude mechanical stimulation as an intervention for osteoporosis. This study will also provide insight into the potential role of muscle adaptation and bone turnover in increasing bone mass in response to this simple, non-invasive, non-pharmacalogical intervention.